ashmem_mutex create a chain of dependencies like so: (1) mmap syscall -> mmap_sem -> (acquired) ashmem_mmap ashmem_mutex (try to acquire) (block)
(2) llseek syscall -> ashmem_llseek -> ashmem_mutex -> (acquired) inode_lock -> inode->i_rwsem (try to acquire) (block) (3) getdents -> iterate_dir -> inode_lock -> inode->i_rwsem (acquired) copy_to_user -> mmap_sem (try to acquire) There is a lock ordering created between mmap_sem and inode->i_rwsem causing a lockdep splat [2] during a syzcaller test, this patch fixes the issue by unlocking the mutex earlier. Functionally that's Ok since we don't need to protect vfs_llseek. [1] https://patchwork.kernel.org/patch/10185031/ [2] https://lkml.org/lkml/2018/1/10/48 Cc: Todd Kjos <tk...@google.com> Cc: Arve Hjonnevag <a...@android.com> Cc: Greg Hackmann <ghackm...@google.com> Cc: Greg Kroah-Hartman <gre...@linuxfoundation.org> Reported-by: syzbot+8ec30bb7bf1a981a2...@syzkaller.appspotmail.com Signed-off-by: Joel Fernandes <joe...@google.com> --- drivers/staging/android/ashmem.c | 15 +++++++-------- 1 file changed, 7 insertions(+), 8 deletions(-) diff --git a/drivers/staging/android/ashmem.c b/drivers/staging/android/ashmem.c index 7e060f32aaa8..c8b74ae53936 100644 --- a/drivers/staging/android/ashmem.c +++ b/drivers/staging/android/ashmem.c @@ -335,24 +335,23 @@ static loff_t ashmem_llseek(struct file *file, loff_t offset, int origin) mutex_lock(&ashmem_mutex); if (asma->size == 0) { - ret = -EINVAL; - goto out; + mutex_unlock(&ashmem_mutex); + return -EINVAL; } if (!asma->file) { - ret = -EBADF; - goto out; + mutex_unlock(&ashmem_mutex); + return -EBADF; } + mutex_unlock(&ashmem_mutex); + ret = vfs_llseek(asma->file, offset, origin); if (ret < 0) - goto out; + return ret; /** Copy f_pos from backing file, since f_ops->llseek() sets it */ file->f_pos = asma->file->f_pos; - -out: - mutex_unlock(&ashmem_mutex); return ret; } -- 2.16.0.rc1.238.g530d649a79-goog
